Various projection lenses for use in video projectors having a liquid crystal display are known in the prior art. These video projectors emit light onto the back surface of a liquid crystal display. The light is then modulated by an image formed on the liquid crystal display and the image of the modulated light is then enlarged by a projection lens and projected onto a screen. One side of the projection lens is termed the reducing side, and it is on this side that the liquid crystal display is to be located. The other side of the projection lens is termed the enlarging side, and it is on this side that an enlarged image of that formed at the liquid crystal display is viewed by being projected onto a screen.
For proper operation of the liquid crystal display, it is necessary for the rays to be incident nearly orthogonally onto the back surface of the liquid crystal display. Thus, to prevent unnecessary loss of light, it is desirable for the projection lens used in this kind of liquid crystal projector to be nearly telecentric on its reducing side. In addition, because the extinction ratio for this type of display is relatively large, it is desirable to employ a lens which forms a bright image, ie., for the lens to have a low F-number. In particular, a lens having an F-number of about 1.4 or less is required.
Japanese Patent Application 7-294809 and U.S. Pat. No. 2,687,063 each disclose a prior art projection lens in which various aberrations are favorably corrected. However, the projection lens disclosed in Japanese Patent Application 7-294809 has a problem in that the F-number value is approximately 2.0, and the maximum angle of a principal ray on the reducing side (hereinafter termed the telecentric angle) is 6.degree. relative to the optical axis. Therefore, this system is not nearly telecentric on its reducing side. Thus, such a lens will result in unnecessary loss of light at the periphery of the image when used with a display which requires the light to be orthogonal to the display surface, as in the case with a translucent liquid crystal display, for example.
On the other hand, the projection lens disclosed in U.S. Pat. No. 2,687,063 yields a brighter image in that it has a lower F-number value of 1.7. However, the telecentric angle is 5.degree. relative to the optical axis. Therefore, this lens also is not nearly telecentric on its reducing side, resulting in extinction of light near the periphery of the lens when used with a display that requires its back side to be illuminated with light that is orthogonal to its surface. Furthermore, certain aberrations, such as distortion, are large. Therefore, this lens is not suitable for use as a projection lens in state-of-the-art projectors.